SPATIAL RELATIONS OF MAJOR TECTONO-IGNEOUS ELEMENTS AND THE ORIGIN OF MAGMAS 



595 



PACIFIC 

 OCEAN 



PACIFIC OCEAN TO COLORADO PLATEAU 



BASIN 



AND 



COAST GREAT 



_ RANGE S VALLEY 



g — _^ — — — — — _ l_'i j.» 7 -"- -i'-"- -"-'-JU-"- *— **~ D.I J l\M/3CW C 11 V 



jftsi—*" 7-0 



40 KM-' "~- 



SIERRA NEVADA 



20 KM'' 



WASATCH 

 PROVINCE MOUNTAINS UINTA BASIN 



-i Tv r ~T~rY]/y^7 - 



633 KM/SEC d KM " 16'TKM-^ 



25 KM 



8 KM/SEC 



50KM 759 KM/SEC 



z»¥"km/sec 



v 



72 KM 



ELKO TO PROMONTORY BLAST SITE 



KILOMETERS 



4200 FT. ABOVE 

 S.L. 



25 KM 



Fig. 38.1. Postulated seismic layering in relation to geologic structures at the surface of the western 

 Cordillera. Velocities in Wasatch and Great Basin from Berg ef al. (1960); for western Great Basin, Press 

 (1960) and in the Sierra Nevada, Gutenberg (1943). 



Tholeiitic Magma 



The occurrence of theoleiitic basalt in eugeosynclines, over a wide area 

 of the older Nevadan batholithic complex, in the Triassic fault basins of 

 the Appalachian Mountains systems, in the Parana basin of the stable 

 shield area of Brazil, and in the Hawaiian Islands of the Pacific basin 

 indicates diat no tectonic setting has a monopoly on the magma. Previous 

 considerations have shown that there is a fairly complete range from 

 olivine basalt to tholeiitic basalt, if world-wide examples are tabulated 

 together, but in local occurrences the spread is usually small, and separate 

 igneous provinces may be recognized. 



Convincing petrographic evidence for the origin of basalt by partial 

 melting of the mantle comes from olivine-rich nodules in basalts. These 

 are concluded by Ross et al. ( 1954 ) to have been xenoliths derived from 

 the peridotitic mantle, and studies by Kuno et al. ( 1957 ) lead them to the 



same theory. Such xenoliths are known from about sixty localities scat- 

 tered throughout the oceanic as well as continental regions. Kuno con- 

 cludes that most of the olivine-rich nodules occur in alkali olivine basalt 

 and allied rocks such as andesine andesite, nepheline or leucite basalt, 

 and basanite and limburgite. Only a few doubtful examples in tholeiites 

 are known. This distribution was first thought to signify that only the 

 alkali olivine basalt comes from the mantle, and that the tholeiitic origi- 

 nates from bodily melting of the basaltic subcrust, but when it was re- 

 alized that the Moho discontinuity under the Hawaiian Islands is only 

 5 kilometers deep, it was concluded that temperatures could not become 

 high enough in the basaltic subcrust to cause melting. Kuno concluded, 

 consequently, that both basalt types originate by partial melting of the 

 upper mantle. 



From extensive petrographic and chemical studies, particularly in 



